以下是CSS中水平居中的解决方案列表。该代码片段包括所有这些。

html {
  font: 1.25em/1.5 Georgia, Times, serif;
}

pre {
  color: #fff;
  background-color: #333;
  padding: 10px;
}

blockquote {
  max-width: 400px;
  background-color: #e0f0d1;
}

blockquote > p {
  font-style: italic;
}

blockquote > p:first-of-type::before {
  content: open-quote;
}

blockquote > p:last-of-type::after {
  content: close-quote;
}

blockquote > footer::before {
  content: "\2014";
}

.container,
blockquote {
  position: relative;
  padding: 20px;
}

.container {
  background-color: tomato;
}

.container::after,
blockquote::after {
  position: absolute;
  right: 0;
  bottom: 0;
  padding: 2px 10px;
  border: 1px dotted #000;
  background-color: #fff;
}

.container::after {
  content: ".container-" attr(data-num);
  z-index: 1;
}

blockquote::after {
  content: ".quote-" attr(data-num);
  z-index: 2;
}

.container-4 {
  margin-bottom: 200px;
}

/**
 * Solution 1
 */
.quote-1 {
  max-width: 400px;
  margin-right: auto;
  margin-left: auto;
}

/**
 * Solution 2
 */
.container-2 {
  text-align: center;
}

.quote-2 {
  display: inline-block;
  text-align: left;
}

/**
 * Solution 3
 */
.quote-3 {
  display: table;
  margin-right: auto;
  margin-left: auto;
}

/**
 * Solution 4
 */
.container-4 {
  position: relative;
}

.quote-4 {
  position: absolute;
  left: 50%;
  transform: translateX(-50%);
}

/**
 * Solution 5
 */
.container-5 {
  display: flex;
  justify-content: center;
}

<main>
  <h1>CSS: Horizontal Centering</h1>

  <h2>Uncentered Example</h2>
  <p>This is the scenario: We have a container with an element inside of it that we want to center. I just added a little padding and background colors so both elements are distinquishable.</p>

  <div class="container  container-0" data-num="0">
    <blockquote class="quote-0" data-num="0">
      <p>My friend Data. You see things with the wonder of a child. And that makes you more human than any of us.</p>
      <footer>Tasha Yar about Data</footer>
    </blockquote>
  </div>

  <h2>Solution 1: Using <code>max-width</code> & <code>margin</code> (IE7)</h2>

  <p>This method is widely used. The upside here is that only the element which one wants to center needs rules.</p>

<pre><code>.quote-1 {
  max-width: 400px;
  margin-right: auto;
  margin-left: auto;
}</code></pre>

  <div class="container  container-1" data-num="1">
    <blockquote class="quote  quote-1" data-num="1">
      <p>My friend Data. You see things with the wonder of a child. And that makes you more human than any of us.</p>
      <footer>Tasha Yar about Data</footer>
    </blockquote>
  </div>

  <h2>Solution 2: Using <code>display: inline-block</code> and <code>text-align</code> (IE8)</h2>

  <p>This method utilizes that <code>inline-block</code> elements are treated as text and as such they are affected by the <code>text-align</code> property. This does not rely on a fixed width which is an upside. This is helpful for when you don’t know the number of elements in a container for example.</p>

<pre><code>.container-2 {
  text-align: center;
}

.quote-2 {
  display: inline-block;
  text-align: left;
}</code></pre>

  <div class="container  container-2" data-num="2">
    <blockquote class="quote  quote-2" data-num="2">
      <p>My friend Data. You see things with the wonder of a child. And that makes you more human than any of us.</p>
      <footer>Tasha Yar about Data</footer>
    </blockquote>
  </div>

  <h2>Solution 3: Using <code>display: table</code> and <code>margin</code> (IE8)</h2>

  <p>Very similar to the second solution but only requires to apply rules on the element that is to be centered.</p>

<pre><code>.quote-3 {
  display: table;
  margin-right: auto;
  margin-left: auto;
}</code></pre>

  <div class="container  container-3" data-num="3">
    <blockquote class="quote  quote-3" data-num="3">
      <p>My friend Data. You see things with the wonder of a child. And that makes you more human than any of us.</p>
      <footer>Tasha Yar about Data</footer>
    </blockquote>
  </div>

  <h2>Solution 4: Using <code>translate()</code> and <code>position</code> (IE9)</h2>

  <p>Don’t use as a general approach for horizontal centering elements. The downside here is that the centered element will be removed from the document flow. Notice the container shrinking to zero height with only the padding keeping it visible. This is what <i>removing an element from the document flow</i> means.</p>

  <p>There are however applications for this technique. For example, it works for <b>vertically</b> centering by using <code>top</code> or <code>bottom</code> together with <code>translateY()</code>.</p>

<pre><code>.container-4 {
    position: relative;
}

.quote-4 {
  position: absolute;
  left: 50%;
  transform: translateX(-50%);
}</code></pre>

  <div class="container  container-4" data-num="4">
    <blockquote class="quote  quote-4" data-num="4">
      <p>My friend Data. You see things with the wonder of a child. And that makes you more human than any of us.</p>
      <footer>Tasha Yar about Data</footer>
    </blockquote>
  </div>

  <h2>Solution 5: Using Flexible Box Layout Module (IE10+ with vendor prefix)</h2>

  <p></p>

<pre><code>.container-5 {
  display: flex;
  justify-content: center;
}</code></pre>

  <div class="container  container-5" data-num="5">
    <blockquote class="quote  quote-5" data-num="5">
      <p>My friend Data. You see things with the wonder of a child. And that makes you more human than any of us.</p>
      <footer>Tasha Yar about Data</footer>
    </blockquote>
  </div>
</main>

display: flex

.container {
  display: flex;
  justify-content: center;
}

备注：

• 这不是黑客👍👍👍
• 浏览器支持：柔性盒

max-width & margin

您可以通过分配固定宽度并将margin-right和margin-left设置为auto来水平居中块级元素。

.container ul {
  /* for IE below version 7 use `width` instead of `max-width` */
  max-width: 800px;
  margin-right: auto;
  margin-left: auto;
}

备注：

• 无需容器👍
• 要求已知居中元素的（最大）宽度👎

IE9+：transform: translatex(-50%) & left: 50%

这类似于使用绝对定位和负边距的古怪居中方法。

.container {
  position: relative;
}

.container ul {
  position: absolute;
  left: 50%;
  transform: translatex(-50%);
}

备注：

• 居中的元素将从 document flow 中删除。所有元素将完全忽略居中的元素。👎👎👎
• 这种技术允许通过使用top而不是left和translateY()而不是translateX()进行 * 垂直 * 居中。这两者甚至可以结合起来。👍
• 浏览器支持：transform2d

IE8+：display: table & margin

就像第一个解决方案一样，您对左右边距使用自动值，但不指定宽度。如果您不需要支持IE7及以下版本，这是更好的选择，尽管为display使用table属性值感觉有点笨拙。

.container ul {
  display: table;
  margin-right: auto;
  margin-left: auto;
}

IE8+：display: inline-block & text-align

像处理常规文本一样将元素居中也是可能的。缺点：您需要为容器和元素本身赋值。

.container {
  text-align: center;
}

.container ul {
  display: inline-block;

  /* One most likely needs to realign flow content */
  text-align: initial;
}

备注：

• 不需要指定（最大）宽度👍
• 将流内容对齐到中心（可能有不需要的副作用）👎
• 可以很好地处理动态数量的菜单项（即如果你不知道一个项目的宽度将占用）👍

制作UL auto的左右边距并为其分配宽度：

#headermenu ul {
    margin: 0 auto;
    width: 620px;
}

**编辑：**正如kleinfreund所建议的，你也可以将容器居中对齐，并给予ul一个inline-block显示，但你还必须给LI一个左浮动或一个inline显示。

#headermenu { 
    text-align: center;
}
#headermenu ul { 
    display: inline-block;
}
#headermenu ul li {
    float: left; /* or display: inline; */
}

ul {
width: 90%; 
    list-style-type:none;
    margin:auto;
    padding:0;
    position:relative;
    left:5%;
}

你可以检查这解决了你的问题...

#headermenu ul{ 
        text-align: center;
    }
    #headermenu li { 
list-style-type: none;
        display: inline-block;
    }
    #headermenu ul li a{
        float: left;
    }

http://jsfiddle.net/thirtydot/VCZgW/

ul {
      text-align: center;
      list-style: inside;
    }